Determining the exact location of a mobile device (e.g. a mobile phone) in an environment can be quite challenging, especially when the mobile device is located in an urban environment, or is located within a building. Imprecise estimates of the mobile device's position may have “life or death” consequences for the user. For example, an imprecise position estimate for a mobile device, such as a mobile phone operated by a user calling 911, can delay emergency personnel response times when responding to the call. In less dire situations, imprecise estimates of the user's position can negatively impact efforts to provide navigation to a desired destination.
Positioning systems used to estimate the position of the mobile device, like the Global Positioning System (GPS), have been in use for many years. Unfortunately, poor signal conditions found in urban and indoor environments may degrade the performance of these conventional positioning systems. To improve positioning accuracy in urban and indoor environments, GPS may be replaced or augmented by positioning systems that use terrestrial beacons, such as those in cellular phone networks, those described in U.S. Pat. No. 8,130,141 and U.S. Pat. No. 9,057,606, or others.
Such systems may transmit positioning/timing signals from multiple beacons of known locations to the mobile device in order to generate pseudoranges that may then be used when generating a position estimate of the mobile device. As is known in the art, a pseudorange may be derived using an estimated time-of-flight of the transmitted positioning/timing signal transmitted by a beacon to the mobile device—i.e., the time during which the signal was in transit between its time of departure from the beacon and the time of arrival at the mobile device. Since the estimated position of the mobile device may be generated using pseudoranges associated with multiple beacons, it follows that the accuracy of the estimated position of the mobile device will be affected by the degree to which the clocks of the multiple beacons are synchronized between themselves.
Unfortunately, synchronization of beacons in terrestrial positioning networks can be difficult or expensive to achieve. For example, a network may rely on providing a time synchronization RF signal to each beacon from a centralized source (e.g. a GNSS, USNO, or another satellite time source). However, some beacons may be unable to receive that synchronization signal from the centralized source, or the cost to provide that synchronization signal from the centralized source to particular beacons is too high. Thus, network-wide and cost-effective approaches for synchronizing a beacon's clock are needed.